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In lecture on Wednesday of week 2, Dr. Lavelle made the comment that tables of bond enthalpies refer to breaking bonds in gases and therefore we need to add enthalpy of phase change for liquids and solids. I'm confused about what this means/how you would do this, can someone explain or give an example?
So when calculating standard enthalpy using bond enthalpies, the mean bond enthalpies provided will be for those bonds in the gaseous phase. In order to calculate the standard enthalpy of a reaction with something in a liquid or solid form, just use the bond enthalpies as you normally would then after you've added/subtracted the bond enthalpies, add/subtract the enthalpy of vaporization or whatever is given. If a reactant is a liquid for example, add the (delta)H of vaporization for that reactant at the end. If a product is a liquid, subtract the (delta)H of vaporization of that product at the end (because for bond enthalpies, breaking bonds is positive and forming bonds is negative).
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